March 02, 2010

Female-to-Male Breeding Ratio in Modern Humans (Labuda et al. 2010)

The American Journal of Human Genetics, 25 February 2010doi:10.1016/j.ajhg.2010.01.029

Female-to-Male Breeding Ratio in Modern Humans—an Analysis Based on Historical Recombinations

Damian Labuda et al.

Abstract

Was the past genetic contribution of women and men to the current human population equal? Was polygyny (excess of breeding women) present among hominid lineages? We addressed these questions by measuring the ratio of population recombination rates between the X chromosome and the autosomes, ρX/ρA. The X chromosome recombines only in female meiosis, whereas autosomes undergo crossovers in both sexes; thus, ρX/ρA reflects the female-to-male breeding ratio, β. We estimated β from ρX/ρA inferred from genomic diversity data and calibrated with recombination rates derived from pedigree data. For the HapMap populations, we obtained β of 1.4 in the Yoruba from West Africa, 1.3 in Europeans, and 1.1 in East Asian samples. These values are consistent with a high prevalence of monogamy and limited polygyny in human populations. More mutations occur during male meiosis as compared to female meiosis at the rate ratio referred to as α. We show that at α ≠ 1, the divergence rates and genetic diversities of the X chromosome relative to the autosomes are complex functions of both α and β, making their independent estimation difficult. Because our estimator of β does not require any knowledge of the mutation rates, our approach should allow us to dissociate the effects of α and β on the genetic diversity and divergence rate ratios of the sex chromosomes to the autosomes.

Long periods of peace with ratios close to 1, and small numbers of episodes where large numbers of men are wiped out in wars and many women are taken by the victors seem like a pretty plausible historic pattern, and is consistent with the lower ratio for East Asia that has had political unity (and hence fewer genocidal wars) for a much longer period of time.

This also reminds me to a statistik about the different chances of men and women to get a mate.

It said, that the most attractive man out of 20 can have 50% of all women if he wants so.Compared to a 75% of men that the most attractive women in a group of 20 can have (I would have guessed 100% LOL)

The difference became bigger at the lower end.

While the least attractive women out of 20, still had a chance with 12% of the males, the least attractive male out of 20 only had a chance with 2% of the females.

For the averages it suposedly was, that an average man can have 1 out of 8 women. While an average women can have 1 out of 3 men.

There is lots of Amazonian violence (and evidence from New Guinea pre-contact is about 1/7th of men are murdered), but it only matters if it impacts their death rate during years when they are having more children more than death in childbirth and from violence impacts women's death rate. Polygny historically has usually been limited to high status inividuals, not a predominant practice.

Where a number of men died in warfare, like the Amazon horticultural and new Guinea Highland groups, it is tempting to immediately assume widespread polygny but in fact in such cultures there if often widespread female infanticide as well has shorter life span for women and shortened life expectancy. Under such conditions, it is the limitation of the number of surviving females that is the critical thing - this serves as an indirect form of population control, keeping these populations from exceeding their carrying capacity too often, and generating larger areas of "no-mansland' between villages, which helps the forest regenerate during the long period of ecological succession need between periods of cultivation (to restore soil fertility).

Ratios of men to women might not actually be that high in cultures with warfare and high male mortality, in other worlds.

High status men with extra wives still occur, but this may have nothing to do with who are actually the fathers of the children these wives have. Polygamous marriages did , however give rise to slightly lower numbers of children per female, at least in my sample in West Africa. More cultural mechanisms limiting population growth???

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